Scorched Air Calc

Wayco Wayne

You dun good. I love jobs like that. There are a lot of houses in my area where the furnaces have been twinned, and are more than double the needed capacity. Homeowners get real nervous when you take out 2 and put back one. It just don't seem right somehow. :) WW

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Grease Monkey

CFM vs BTU

I'm replacing an oil fired, forced air furnace for a client and they've complained about not enough air at the end of the duct runs. I did find that there wasn't enough return ducting so I'm going to take care of that. My question is how do I convert the BTU's I need for each room in my heat loss calculation to cfm for each space? The furnace I'm installing has a 70 degree rise.

[Deleted User]

Each CF

will carry .018 btus per hour per degree F differential.

So, (70 X delivered CFM) X (.018 X 60) should equal btuH. I'm sure one of the better learned folks here who deal with air on a regular basis may have an easier/better answer though...

ME

Tony_8

generally

on a 70 deg rise you can figure about 6000 BTUH/6" run. 6" is good for 100 CFM, 8" = 210CFM. That's at a nice easy FPM velocity that won't be too noisy. Balance on air systems is quite dependent on length, sizes and design(ells,etc.). Basically, you're pushing a rope. Keep that in mind and it'll work easier.

Don_2

With out

going into great detail,on proper duct design,I will just
keep it simple,btu Divided by thirty will get you there.
But if you are looking for more then rule of thumb and
you would like to do a better job for your customer,made I
suggest Manual d for residential duct system,from acca.
good luck.

Steve Ebels

To solve for CFM:

> I'm replacing an oil fired, forced air furnace

> for a client and they've complained about not

> enough air at the end of the duct runs. I did

> find that there wasn't enough return ducting so

> I'm going to take care of that. My question is

> how do I convert the BTU's I need for each room

> in my heat loss calculation to cfm for each

> space? The furnace I'm installing has a 70 degree

> rise.

Steve Ebels

To solve for CFM:

You find the product of 1.08 X delta T. (Let's use 60* temp rise). So we have 64.8. Now from your heat loss, take your btu requirement, (let's say 8,500 for example) and divide it by your temp factor. 8,500/64.8 = 131.7 CFM.

Moral of the story....... you have to know what delta T you are working with and your btu requirement. The rest is a piece of cake.

Hope this helps

hot_rod

all good answers

But if you know the model number of the furnace and have access to the tech sheets, you can see the actual CFM's the unit is capable of achieving at varying duct friction factors.

Knowing the CFM capacity can then be utilized to determine the ductwork capacity as it steps down along its length. Many of the duct calculators have friction loss values for each type of fitting or transition.

Takes time to get the precise "book" values. We have a Balnor hood that can read actual CFM's, which can be quite a time saver.

Which reminds me of one time a customer complained of low airflow - sometimes. That too was at the end of the trunk run and a 7" round run to the room from there. The damper set indicated it was wide open and the wing nut was tight. Once I dismantled the 7", the problem was crystal clear. The damper had come loose and had been blown down-line to the riser 90. Evidently, it would sometimes flip over and block off flow. Falls into the expect the unexpected category!

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Tom Eli

Forced Air Savvy.

Many inherent problems with poor design ductwork distribution systems can be mitigated via the use of 2 stage furnaces with variable speed blowers. Granted, correcting ductwork is ultimate soloution, however sometimes not accessable in multi story homes, etc.
On low fire a reduced blower speed makes for a longer not so harsh heating cycle. The circulation through the structure mixes the air, helping to achive more even temperatures within the envelope, and particuarly between different levels.
Most of the heating season the unit will not come out of the low stage. The high fire and blower are available upon increased demand. A two stage stat should be installed for full benifits, however the existing single stage stat can be used in a timing scenario.
The variable speed blower feature can move that air against high static situations via dip switch settings. The resistance is "seen" and the blower ramps up to compensate.
Continous fan on a programmable stat can also be set up to keep different levels from "running away" from each other during the time a stat is not calling. Helps also to constantly filter the air.
This scenario does wonders in 30 year old split level homes with scorched air that have never been comfortable.
I like Hydronics better, but have been able to make some real nice improvements in forced air comfort using equipment described above without gutting the house or clients budget.
The fixed limits in todays equipment force you to size to the existing ductwork and yes, proper return air is more critical than ever before.
"Scortched Air" as a term signifies a thing of the past. With utilization of state of the art furnaces, and a savvy application, many sins can be mitigated.
Best Wishes,
Tom Eli

Tom Eli

Forgive Me.

My rambling above would apply to GAS, not oil.
I Re read the thread and realized ...
Tom Eli

Don_2

Thanks steve

but I was assuming he was designing for ac too,but that was not what he was asking,sorry.
I guess there are still some home's in certain parts of the country that don't require ac.

Darin Cook_2

Very Simple

I was in Home Depot and they have a beautiful work sheet to figure out cfm needs for each room. Of course it is for heating and cooling combined and does not take into account any kind of air exchanges or building construction (or any other boring details). But it looked so simple like any average joe homeowner could easily figure out the duct needs for a house. Funny too how there were no containers of duct sealant.

GW

rule of thumb?

I thought I'd babble a bit, this may or may not pertain to any of the above...

Delta T x 1.08 x cfm = BTU. Manual J is great ( I use Elite Manual J 8th edition), Manual D is cool, ductulators are neat too, but what about leakage? What about 120* attic temp? What about 5" thick filter media? It's all a calculated guess until you measure the air and temps and the pressure drops.

Here's a good start- measure the static pressure at the furnace, just before and just after the blower, add the two, and that's the TESP (total external static pressure). Now, look at the manuafcturer's chart on the blower's performance- this is what the unit is blowing.

Also, you can do a amp reading and calculate the same.

Just because the outdoor unit has a "36" somewhere in the model number doesn't mean the system is moving 1200 cfm!

Once you measure the supply and returns for cfm and delta T, you now know what's leaking.

I learned some cool stuff with NCI, check them out. I can now diagnose a system in a matter minutes, and let the owner know whether I can save them some big, small, or no dollars in their fuel/electric costs. Without measuring, it's just a good or bad guess.

Gary


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MikeB34

Oversized

I did a similar replacement with similar problem of poor air circulation to the farthest reaches. I sized the house (shell calc only) and found the load to be about 56000 BTU/h. The original conversion burner was 120,000!!! The problem was the heat would trip the stat b4 the heat reaches the end of the run! Replaced it with a 60,000 mid eff gas furnace. The new unit ran perfectly all last winter (1 of the coldest we ever had) no discomfort, and all the rooms nice and toasty. Those old installs are way(!!) oversized. Check your load. The ducts may be fine. (Also his gas bill dropped so much the utitlity checked his meter 3x to see if it was accurate, or if he was somehow stealing gas!) Good luck!

MikeB34

Thx

I like those too, That was 1 of many like that (way oversized). And he thretened to move in to my place if it got to cold in his, but the referals are pouring in now. this guy told everyone how much I saved him (no $2000.00 tip tho);-}